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1982 Mercedes 300D Turbo Diesel
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141 Posts
Discussion Starter · #1 ·
The tachometer in my 82 300D hasn't work since I acquired her a few months ago. It occasionally will jump around but it's completely random.

I discovered yesterday that if I honk the horn (actually my 2 year old daughter did, she was sitting in the driveway honking the horn) the tachometer jumps up to full revs. Could this be related to an electrical or vacuum issue that the horn is somehow activating? It happens every time the horn in honked.

I'm hoping it's just a vacuum leak that I can fix.
 

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1985 300D W123 Cali
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1,409 Posts
Sounds more like a ground issue. I found that I could eliminate a few crazy issues with my instruments by adding a ground strap from the back of the console to the frame. I attached at one of the screws on the speedometer.

This isn't to say that you don't have a bad tach amp, like many of these cars have.
 

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2013 MINI Copper S Clubman, '84 300CD-weekend car
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10,152 Posts
^^Agreed, it's not a vac issue.
I don't understand the ground thing tho, as the entire cluster is surrounded by rubber, so I would think the necessary grounds are through the various plugs.
Yet I don't see how adding a ground can hurt. You can use any of the metal screws on the back.
 

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1985 300D W123 Cali
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1,409 Posts
^^Agreed, it's not a vac issue.
I don't understand the ground thing tho, as the entire cluster is surrounded by rubber, so I would think the necessary grounds are through the various plugs.
Yet I don't see how adding a ground can hurt. You can use any of the metal screws on the back.
The back of the cluster has various wires running within it not unlike a PCB. Sometimes the connections get burned and don't conduct back to the circular plug on the oil/temp/gas part of the cluster. by adding a new path to ground, many of the electrical gremlins will go away.
 
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